1. Field of the Invention
This invention relates to a piston for internal combustion engines, comprising a cover layer which has been applied to the piston head by plasma spraying or flame spraying and comprises a material having a relatively low thermal conductivity, preferably .lambda..ltoreq.2 W/mK.
2. Discussion of Prior Art
The requirements for lower fuel consumption and lower emission of polluants in the exhaust gases of internal combustion engines have resulted, for example, in diesel engines, in the use of a higher brake mean effective pressure (output, torque). This has been accomplished in many cases by the provision of an exhaust-driven supercharger. The high output of the engine per unit of displacement results in a high heat loading of the piston requiring increased cooling of the piston to maintain the piston's stength and function. Increased cooling is, however, inconsistent with the requirement to reduce the dissipation of heat generated by the combustion process to the coolant and to the lubricating oil. The latter requirement is due to the following reasons:
A. If the quantities of lubricating oil and of coolant and the size of the radiator can be reduced, less power is required to drive the fan. PA1 B. The higher exhaust gas temperature can be utilized in the supercharged engine to reduce the fuel consumption. PA1 C. The improved evaporation of the fuel spread over the wall of the hot piston head improves the quality of the exhaust gas, particularly when the engine is warming up. PA1 .lambda.=thermal coefficient PA1 W=heat flow (Watt) PA1 m=length (meter) PA1 K.=temperature (Kelvin)
On the other hand, by decreasing the extent to which heat is dissipated by the coolant, a high heat loading of the piston head results so that the piston must be heat-insulated.
Various kinds of insulated piston heads have been proposed. For instance, an aluminum piston having a screw-connected ceramic head which is insulated from the skirt by steel discs has been described and investigated by J. H. Stang in "Designing Adiabatic Engine Components, SEA 780,069. A temperature of about 900.degree. C. can be reached at the piston head of such aluminum piston. However, the so-called hot piston which results is obtained only by the use of a ceramic top having the required strength, such ceramic tops are expensive, additionally the volume of the dead space disposed above the first piston ring is relatively large so that the composition of the exhaust gas can be adversely affected.
It is also known to heat-insulate the surface of the piston head from the body of the piston for an internal combustion engine by providing a protecting cover layer containing zirconium oxide, zirconium silicate, cermets or the like. These can be applied by plasma or flame spraying in a thickness of 0.5 to 3 mm to provide a covering layer having a peak-to-valley height of 50 to 100 .mu.m.
An important disadvantage of that heat-insulating cover layer resides in that when a sufficiently thick covering layer is applied to the piston, the layer's adhesion to the materials which constitute the body of the piston is not ensured under all loading conditions so that the cover layer has a sufficiently long life before the cover layer separates from the body of the piston. This is due to the high heat loading of the cover layer, particularly to the frequent changes of temperature, by which the cover layer is gradually weakened resulting in the formation of cracks. This is also due to the large temperature gradient which is set up in the cover layer giving rise to correspondingly high stresses.